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Wilson Cheung
EE 310
Experiment 3 Formal Report
Introduction
In this lab, we were to design a power supply based on a set of criteria. These were having the power
supply have 120V 60Hz AC input and outputting -15V DC with a 0.5 V tolerance. In order to
simplify the design process, the circuit was broken down into the following sections: Power
Transformer, Rectifier, Filter Capacitor, and Voltage Regulator. It was also required to have the
nominal voltage into the regulator be -24V DC, have a maximum ripple voltage of 3.6v (15%) at
the regulator input, and be rated to provide 1w to the load. Components were used that were
rated to operate twice the calculated values.
Circuit Design
Power Transformer- 12:1:1 Center Tapped Transformer.
Figure 1
RectifierTable 1
Diode Part Number Rated Peak-Inverse-Voltage Average Current Rating Peak Current Rating
1N4004
400 V
1.0 A
30 A
1N4934
100 V
1.0 A
30 A
1N4935
200 V
1.0 A
30 A
Filter Capacitor- Electrolytic with any of the following capacitance:
47µF, 100µF, 220µF, or 470µF rated at 50V DC working voltage 1000µF
rated at 16v DC working voltage
Voltage Regulator- Two different types of regulators were to be built and compared. The first was a
Zener Diode regulator using any of the diodes. The second was an IC regulator using any of the IC’s.
Table 2
Diode Part Number Zener Voltage
1N4728
3.3 V
1N4732
4.7 V
1N4735
6.2 V
1N4736
6.8 V
1N4738
8.2 V
1N4739
9.1 V
1N4742
13 V
Table 3
IC Regulator Part Number Input Voltage Range Typical Output Voltage
LM7805
7.5 V – 20 V
5V
LM7815
17.5 V – 30 V
15 V
LM7905
-20 V – -7 V
-5 V
LM7915
-30 V – -17.5 V
-15 V
Power Transformer
Since there was only one transformer available to us it made for an easy decision when designing this
part of the power supply. However, the transformer does have a small internal winding resistance which
must be taken into account.
Figure 2
12:1:1 transformer connected to 120 Vrms source with output VAB = 10 Vrms, VBC = 10 Vrms, and VAC = 20
Vrms.
Rectifier
The input to the voltage regulator needed to be -24 V so the output must have been greater or equal to
this. The power transformer we chose to use will provide 20 Vrms or about 28.3 Vp.
Figure 3
The maximum voltage at Vout is the peak AC voltage minus the voltage across any diodes that are on.
There are going to be two diodes on that can be represented by a constant voltage drop of VD = 1 V.
Since the peak AC voltage is 28.3 V, this gives a maximum Vout of 26.3 V. RL should be absorbing 1 watt
which will require a resistance of 576 Ω. Also, since we want a negative voltage, setting the positive
terminal as ground will give us the negative output.
Filter Capacitor
We needed to reduce the ripple voltage to meet the specifications of the power supply. This was done
by adding a capacitor in parallel with RL. Since C and Vr are inversely related using the largest allowable
Vr (3.6v) will give us our smallest possible C value of 105.7 µF. A 100 µF capacitor will give an average Vout
of -24.5 V which is much closer to the desired Vc than it would be if using the 220 µF.
Supporting Analysis
Power Transformer
12:1:1 Transformer means Vin/12 = VAB/1 = VBC/1 -> VAB = VBC = 120Vrms/12 = 10 Vrms
VAC = VAB + VBC = 10 Vrms + 10 Vrms = 20 Vrms
Rectifier
Vpeak = Vrms * √2= 20 * √ 2= 28.28 Vpeak
Vout max = Vpeak – 2 * VD = 28.3 V – 2 * 1 V = 26.3 V
RL = (VRL)2/PRL = (Vc)2/PRL = (-24 V)2/ 1 W = 576 Ω
PIV = Vpeak = 28.28 V
Voltage Regulator
Filter Capacitor
Data
Power Transformer
When measuring the open circuit voltages of the power transformer we found that VOC of
AB was 9.9 Vrms, VOC of BC was 9.9 Vrms, and VOC of AC was 20.4 Vrms. By connecting a 432 Ω
test resistor across AC and measuring a Vtest of 20.15 we were able to calculate the internal
winding resistance to be 2.68 Ω.
Rectifier
After finding the internal resistance of the transformer was known we built and connect the
rectifier portion of the circuit. Once the circuit was built we connected it to the oscilloscope
to ensure that the rectifier was working correctly.
Figure 4
The circuit was able to rectify the AC input to our -27 V DC component. Since there is no
filter capacitor installed, the voltage has an ripples.
Figure 5
From the measurements in the figure, the ripple voltage at VC is max – min = 4.2 V. When
comparing this voltage to the input from the transformer, the rectifier and filter capacitor
were converting the AC source into DC.
Figure 6
There is still some ripple so next we attach the voltage regulator.
Figure 7
The ripple voltage has practically been eliminated by using this regulator. The graphs for the
waveforms from the regulator with and without a load attached were the same. When
measured with the DMM, the open circuit voltage was -14.92 V while the voltage across the
load was -14.73. This gave us a % regulation of 1.27%.
Discussion
The output of both voltage regulators stayed well within this range for a variety of different
loads. For the voltage into the regulator input, the specification was -24 V while in our circuit it
ranged from -23.6 V to -27.8V with the ripple. With a maximum ripple of 3.6 V, the voltage into
the regulator falls in the specified range. For the ripple, the input at Vc had a ripple of 4.2 V
which was a little high. This is most likely caused from using a filter capacitor that was slightly
smaller than the calculated value. This can easily be corrected by using the next larger size
capacitor. All of the components that were selected to be used were rated for conditions well
over what would be seen in the circuit.
Summary and Conclusion
This experiment gave an insight on how to convert AC power to DC power. This is useful as a lot
of electrical devices have a circuit. The input 120 V AC and being able to convert that to DC
allows us to work with a wider range of inputs when designing a circuit. Voltage regulators can
be helpful when working with components that are sensitive to fluctuations. This shows
another method of creating a buffer circuit other than the op-amp buffer circuit.